How to make it

Crust: Beat the butter in a medium bowl with an electric mixer on medium-high until smooth. Add the sugar and salt and continue to beat until evenly combined. Scrape down the sides of the bowl, then beat in the egg yolk. Add half of the flour, beating until just crumbly. Scrape down the bowl again; add the remaining flour and then the sour cream or yogurt, beating just until the dough is evenly moistened. Turn dough onto a lightly floured work surface and knead lightly to bring it together.

Roll the dough between 2 generously floured sheets of waxed or parchment paper into a circle about 1/4-inch thick with a rolling pin. Refrigerate for 1 hour.

Cut out 6 (4 1/2-inch) rounds using an inverted bowl or round cookie cutter. Place rounds in the muffin tins and, using a small shot glass or your fingers, press into the corners and about halfway up the sides for a snug fit (see photo). Freeze dough in the muffin tin for 30 minutes.

Position a rack in the lower third of the oven and preheat to 325 degrees F.

Spray the outside of 6 standard muffin liners with cooking spray and place in the crusts. Fill with dried beans or pie weights. Bake crusts until just brown around the edges, about 25 to 30 minutes. Remove from the oven and cool for about 1 minute; then carefully remove the muffin liners and baking beans. Return pan to the oven and continue to bake until crusts are cooked through and evenly browned (see photo), about 15 to 20 minutes more. Cool slightly. Then carefully remove crusts from the muffin tin and cool completely on a rack.

Lemon Filling: Combine the sugar, cornstarch, and yolks in a nonreactive saucepan. Whisk constantly over medium-low heat until the mixture is smooth and sugar dissolves, about 1 minute. Add the lemon and lime juice and zest and continue to cook, whisking constantly, until the mixture is as thick as sour cream and is just about to simmer, 3 to 4 minutes. (Take care to stir into the sides of the pot so that all curd thickens evenly.)

Strain through a fine mesh strainer into a bowl. Whisk in the butter a little at a time, until smooth. Stir occasionally until cooled. (Setting the bowl in a larger bowl of ice will speed this up.). Fill each tartlet with 1 1/2 tablespoons of the cool lemon filling.

Meringue: Bring a few inches of water to a boil in a saucepan that can hold a standing mixer's bowl above the water. Whisk together the egg whites, sugar, cream of tartar and salt in the bowl by hand. Set the bowl above the boiling water and continue whisking until the mixture is hot to the touch (135 degrees F) and the sugar dissolves, about 1 to 2 minutes. Transfer the bowl to the mixer fitted with the whisk attachment and beat the whites at medium-high speed until they hold soft peaks. Increase speed to high and continue to beat to make a stiff, cool meringue, about 10 minutes. Dollop or pipe on top of the filling.

Just before serving, preheat the broiler to high. Set the pies on a baking sheet, and place under the broiler until the meringue is evenly toasted, about 2 minutes. (Alternatively, brown meringue with a blowtorch.) Serve immediately or refrigerate until ready to serve.

For Busy Bakers:

They can be refrigerated for up to 2 days These also freeze wonderfully! They can be assembled and frozen up to 1 week in advance. Defrost for 20 minutes before broiling the tops and serving.